Output of
Query 1:
select id from users
order by case when DEVIATION_LEVEL=2863 then 1 else 2 end
is
800019
800030
800040
800003
800007
800015
800025
800026....etc
Output of
Query 2:
select id from
(select id from users
order by case when DEVIATION_LEVEL=2863 then 1 else 2 end)
where rownum<=16;
is
800019
800030
800028
800020
800021
800018
800012
800161...etc
Why the order changes in 2nd query? Please suggest correct solution to just limit the size of the first query result.
When you perform a SELECT query without an ORDER BY clause the order of the result is undetermined. If you want or need to have a consistent ordering behavior then use the ORDER BY clause at the top level SELECT.
There is however the exception in oracle when you are limiting the rows with the ROWNUM field. In that case the ROWNUM filter would reduce the result set before applying the order by clause, thus removing rows that should have come first.
select id from users
order by case when DEVIATION_LEVEL=2863 then 1 else 2, id;
and
select id from
(select id from users
order by case when DEVIATION_LEVEL=2863 then 1 else 2 end, id)
where rownum<=16;
The reason is that ORDER BY cannot guarantee the ordering on duplicate values.
In your query, put the DEVIATION_LEVEL in the column list of the select and then you will understand that the order is just not same when they are duplicate values.
For example,
Query 1
SQL> SELECT empno, deptno FROM emp ORDER BY CASE WHEN deptno=10 THEN 1 ELSE 2 END;
EMPNO DEPTNO
---------- ----------
7782 10
7839 10
7934 10
7566 20
7654 30
7698 30
7900 30
7788 20
7369 20
7844 30
7876 20
7521 30
7499 30
7902 20
14 rows selected.
Query 2
SQL> SELECT empno, deptno
2 FROM
3 (SELECT empno, deptno FROM emp ORDER BY CASE WHEN deptno=10 THEN 1 ELSE 2 END
4 )
5 WHERE rownum<=5;
EMPNO DEPTNO
---------- ----------
7782 10
7934 10
7839 10
7369 20
7654 30
SQL>
So, the ordering in the second case when ROWNUM is applied, it is picked randomly among the similar values.
Look at the first three ordered rows:
Output 1
EMPNO DEPTNO
---------- ----------
7782 10
7839 10
7934 10
Output 2
EMPNO DEPTNO
---------- ----------
7782 10
7934 10
7839 10
ORDER BY deptno will not guarantee same order every time. In above query, if you want a particular order, then make an ORDER BY on another column too, i.e. empno.
ORDER BY empno, deptno
If you compare both the outputs, there is no guarantee that ordering will be same always since the deptno is same as 10 in all three rows. When you have similar values, and if you order them, it is just like picking them in random.
ORDER BY in a subquery does not guarantee the results; SELECT * FROM table ORDER BY 1; is not same as SELECT * FROM (SELECT * FROM table ORDER BY 1);
Related
I was trying to print odd numbers of rows from my table without taking taking help of my numeric cloumns
when I try to execute this query I was getting only first row.
select * from emp3 where mod(rownum,2)=1;
emp3 is my table name.
and when I use my one of the numeric columns in place of rownum I was getting desired output.
select * from emp3 where mod(eid,2)=1 order by eid;
where eid is a numeric column in the table.
But what if do not have a numeric column and I want to print only odd number of rows from the table?
Help me!
Try to execute the below query
select * from (select rownum rn ,column from column_name) where mod(rn,2) <> 0
and please refer to this link for better understanding the concept of rownum https://www.youtube.com/watch?v=QMyw1jumGyQ
If the EID column isn't numeric, then use something that is. For example, ROW_NUMBER gives such an information:
SQL> with temp as
2 (select empno, ename, job sal,
3 row_number() over (order by null) rn
4 from emp
5 )
6 select *
7 from temp
8 where mod(rn, 2) = 1;
EMPNO ENAME SAL RN
---------- ---------- --------- ----------
7369 SMITH CLERK 1
7521 WARD SALESMAN 3
7654 MARTIN SALESMAN 5
7782 CLARK MANAGER 7
7839 KING PRESIDENT 9
7876 ADAMS CLERK 11
7902 FORD ANALYST 13
7 rows selected.
SQL>
Or even ROWNUM you already tried to use:
SQL> with temp as
2 (select empno, ename, job sal,
3 rownum rn
4 from emp
5 )
6 select *
7 from temp
8 where mod(rn, 2) = 1;
EMPNO ENAME SAL RN
---------- ---------- --------- ----------
7369 SMITH CLERK 1
7521 WARD SALESMAN 3
7654 MARTIN SALESMAN 5
7782 CLARK MANAGER 7
7839 KING PRESIDENT 9
7876 ADAMS CLERK 11
7902 FORD ANALYST 13
7 rows selected.
SQL>
currently I am using the below query to get the previous year’s bonus amount for the employees. But I am facing some issues, so I am trying to get the latest element entry value(screen entry value) for the element ‘xyz bonus’ using the RANK() function. Please help. Thanks.
Select
Pam.assignment_number,
Peev.screen_entry_value as bonus_amount
From
Per_all_assignments_m Pam,
Pay_element_entries_f peef,
Pay_element_types_tl petl,
Pay_element_entry_values_f peev
Where
Pam.Person_id=peef.person_id
and peef.element_type_id = petl. element_type_id
And peef.element_entry_id = peev. element_entry_id
And petl.language=‘US’
And to_char(peef.effective_start_date,’yyyy’)=(to_char(sysdate,’yyyy’)-1)
And to_char(peev.effective_start_date,’yyyy’)=(to_char(sysdate,’yyyy’)-1)
And petl.element_name = ‘xyz bonus’
As I don't have your tables, I'm using Scott's sample EMP table.
In there, rows sorted by salaries per department look like this:
SQL> select deptno,
2 ename,
3 sal,
4 rank() over (partition by deptno order by sal desc) rn
5 from emp
6 order by deptno,
7 sal desc;
DEPTNO ENAME SAL RN
---------- ---------- ---------- ----------
10 KING 10000 1
10 CLARK 2450 2
10 MILLER 1300 3
20 SCOTT 3000 1
20 FORD 3000 1
20 JONES 2975 3
20 ADAMS 1100 4
20 SMITH 920 5
30 BLAKE 2850 1
30 ALLEN 1600 2
30 TURNER 1500 3
30 MARTIN 1250 4
30 WARD 1250 4
30 JAMES 950 6
14 rows selected.
SQL>
If you want to fetch the highest salary per department, you'd then
SQL> select deptno, ename, sal
2 from (select deptno,
3 ename,
4 sal,
5 rank() over (partition by deptno order by sal desc) rn
6 from emp
7 )
8 where rn = 1;
DEPTNO ENAME SAL
---------- ---------- ----------
10 KING 10000
20 SCOTT 3000
20 FORD 3000
30 BLAKE 2850
SQL>
I guess that's what you are looking for.
Your query might then look like this:
Select
Pam.assignment_number,
Peev.screen_entry_value as bonus_amount,
rank() over (partition by pam.assignment_number order by peev.screen_entry_value desc) rn
From
...
Now, use it as an inline view (or a CTE) and fetch desired values.
If that's not what you are looking for, please, post sample data and desired result.
This is a common question I saw in many places, but don't know yet it possible or not. I'm trying to select rows between 2 and 5 in following way using oracle sql developer tool.
As of result this query, this should select 3rd and 4th query according to below query
SELECT * FROM MyTable
WHERE ROWNUM > 2 AND ROWNUM < 5
but it's not selecting the 3rd and 4th rows,
Then I tried the following query
SELECT * FROM MyTable
WHERE RN BETWEEN 2 AND 5
This also syntactically and progrmatically correct, but not selecting the exact columns.
Use a subquery:
SELECT t.*
FROM (SELECT t.*, ROWNUM as rn
FROM MyTable t
) t
WHERE rn > 2 AND rn < 5;
Note that tables represent unordered sets. There is no such thing as a first or second row. You should have an ORDER BY clause to specify the ordering.
The reason that your version doesn't work is that rownum starts at 1 when the first row is put into the result set. If no row is put in, the value never increments. So, it never hits 2 or 3.
I should also note that between in SQL is inclusive. So >= and <= are more appropriate.
EDIT:
I should note that Oracle 12+ supports FETCH/OFFSET:
select t.*
from mytable t
offset 2 -- start on the third row
fetch first 2 rows only -- fetch two rows in total
An order by is still recommended in this case.
A little bit of analytics.
Salaries in the EMP table, sorted by $$$, look like this:
SQL> select ename, sal
2 from emp
3 order by sal;
ENAME SAL
---------- ----------
SMITH 800
JAMES 950 2 you want to return James ...
WARD 1250 3
MARTIN 1250 4
MILLER 1300 5 ... to Miller
TURNER 1500
ALLEN 1600
CLARK 2450
BLAKE 2850
JONES 2975
FORD 3000
KING 5000
12 rows selected.
SQL>
If you do it as follows, you'd get what you wanted:
SQL> select ename, sal, rn
2 from (select ename, sal, row_number() over (order by sal) rn
3 from emp
4 )
5 where rn between 2 and 5;
ENAME SAL RN
---------- ---------- ----------
JAMES 950 2
WARD 1250 3
MARTIN 1250 4
MILLER 1300 5
SQL>
However, as you can see, Ward and Martin earn the same $1250. So, should we count them as having the same salary and include Turner into the list, or not? Yet two another analytic functions might help you decide: RANK and DENSE_RANK:
SQL> select ename, sal,
2 row_number() over (order by sal) rn,
3 rank() over (order by sal) rnk,
4 dense_rank() over (order by sal) drnk
5 from emp
6 order by sal;
ENAME SAL RN RNK DRNK
---------- ---------- ---------- ---------- ----------
SMITH 800 1 1 1
JAMES 950 2 2 2 2nd isn't questionable, but ...
WARD 1250 3 3 3
MARTIN 1250 4 3 3
MILLER 1300 5 5 4 ... which one is 5th? Miller (RN and RNK), ...
TURNER 1500 6 6 5 ... or Turner (DRNK column)?
ALLEN 1600 7 7 6
CLARK 2450 8 8 7
BLAKE 2850 9 9 8
JONES 2975 10 10 9
FORD 3000 11 11 10
KING 5000 12 12 11
12 rows selected.
SQL>
To be fair, DENSE_RANK is probably the best option in such cases:
SQL> select ename, sal, drnk
2 from (select ename, sal, dense_rank() over (order by sal) drnk
3 from emp
4 )
5 where drnk between 2 and 5;
ENAME SAL DRNK
---------- ---------- ----------
JAMES 950 2
WARD 1250 3
MARTIN 1250 3
MILLER 1300 4
TURNER 1500 5
SQL>
Now you have several options; pick the one that suits you best.
What to do when we want to select salary of a employee greater than many (lets say 12) employees's salary from a table. I know that we'll have to use a subquery but writing it as :-
Select ename,salary
from emp
where salary>( select salary
from emp
where ename='A'||ename='B'.....)
it could be written like that but its not a good approach. Please suggest some useful query for it.
If you know the 12 employees, I think you want to write the query as:
Select ename,salary
from emp
where salary> (select max(salary)
from emp
where ename in ('A', 'B', . . . )
)
IN is much more convenient than a bunch of or statements. And, the subquery needs to return one value, the maximum salary.
Select ename,salary
from emp
where salary > (
select salary
from
(
select
salary,
rownum as rn
from emp
order by salary
)
where rn = 12
)
This is not exact code that you may use, but it should help you.
You can use RANK() function.
Example from article at oracle-base.com:
SELECT empno,
deptno,
sal,
RANK() OVER (PARTITION BY deptno ORDER BY sal) "rank"
FROM emp;
EMPNO DEPTNO SAL rank
---------- ---------- ---------- ----------
7934 10 1300 1
7782 10 2450 2
7839 10 5000 3
7369 20 800 1
7876 20 1100 2
7566 20 2975 3
7788 20 3000 4
7902 20 3000 4
7900 30 950 1
7654 30 1250 2
7521 30 1250 2
7844 30 1500 4
7499 30 1600 5
7698 30 2850 6
I can see two different interpretations of your requirement.
1. What employees earn more than 12 other (random) employees
and
2. What employees earn more than 12 specific employees
This query solves the first requirement, although it will become slow as hell on larger datasets.
select *
from emp a
where 12 = (select count(*)
from emp b
where b.salary < a.salary);
This query solves the second requirement
select *
from emp
where salary > all(select salary
from emp
where emp_id in(1,2,3,4,5)
)
What is the difference between ROWNUM and ROW_NUMBER ?
ROWNUM is a "pseudocolumn" that assigns a number to each row returned by a query:
SQL> select rownum, ename, deptno
2 from emp;
ROWNUM ENAME DEPTNO
---------- ---------- ----------
1 SMITH 99
2 ALLEN 30
3 WARD 30
4 JONES 20
5 MARTIN 30
6 BLAKE 30
7 CLARK 10
8 SCOTT 20
9 KING 10
10 TURNER 30
11 FORD 20
12 MILLER 10
ROW_NUMBER is an analytic function that assigns a number to each row according to its ordering within a group of rows:
SQL> select ename, deptno, row_number() over (partition by deptno order by ename) rn
2 from emp;
ENAME DEPTNO RN
---------- ---------- ----------
CLARK 10 1
KING 10 2
MILLER 10 3
FORD 20 1
JONES 20 2
SCOTT 20 3
ALLEN 30 1
BLAKE 30 2
MARTIN 30 3
TURNER 30 4
WARD 30 5
SMITH 99 1
From a little reading, ROWNUM is a value automatically assigned by Oracle to a rowset (prior to ORDER BY being evaluated, so don't ever ORDER BY ROWNUM or use a WHERE ROWNUM < 10 with an ORDER BY).
ROW_NUMBER() appears to be a function for assigning row numbers to a result set returned by a subquery or partition.
Apart from the other differences mentioned in answers, you should also consider performance. There is a non-authoritative but very interesting report here, comparing various means of pagination, among which the use of ROWNUM compared to ROW_NUMBER() OVER():
http://www.inf.unideb.hu/~gabora/pagination/results.html
rownum is a pseudocolumn which can be added to any select query, to number the rows returned (starting with 1). They are ordered according to when they were identified as being part of the final result set. (#ref)
row_number is an analytic's function, which can be used to number the rows returned by the query in an order mandated by the row_number() function.
Rownum starts with 1 ..increases after condition evaluated results to true .
Hence rownum >=1 returns all rows in table